The present invention relates generally to electrophotographic printing and, more particularly, to an improved transfer-roll system and method for enhanced printing, digital imaging, lithographic reproduction or the like.
During electrophotographic printing, for instance, a printed image is typically produced by transferring toner from a rotating, cylinder-like photoconductor to a sheet of paper or similar medium. This is usually accomplished by placing the paper against the photoconductor at a selected transfer point. A corona then applies a charge, such as by spraying, onto selected portions of the back side of the paper, the charge causing the paper to attract toner to the corresponding charged portions of the paper. As the photoconductor is rotated, i.e., during the toner transfer step, the paper is moved in tandem with the photoconductor. Upon contact between the photoconductor and the paper, the toner image corresponding to the charged portions is transferred from the photoconductor to the paper.
Other conventional electrophotographic printers use a semiconductive transfer-roller which is placed against the back of the paper for attracting toner to the front thereof. The roller is also soft, allowing it to press gently and uniformly against the rotating photoconductor, thereby squeezing the paper evenly against the photoconductor. Simultaneously, a voltage is applied to the roller for attracting and retaining toner on the paper. Because the soft roller, by its nature, is constructed of materials that are porous, it often captures and retains toner from the photoconductor. This usually occurs at locations where there is no paper between the roller and photoconductor.
To prevent toner and corresponding portions of the desired image from coming into contact with the transfer-roller, applications software has typically been provided to insure that the image is kept away from edge portions of the paper. In addition, transfer-rollers have been utilized that are of the same or greater width than the paper being printed upon. Since the photoconductor touches the roller only where no printing occurs, it was, therefore, believed that little or no toner could be developed on the transfer-roller.
To the contrary, however, some toner often ends up on the transfer-roller, such as on lengthwise or gap regions between consecutive sheets of paper. Toner transfer also occurs when printing on envelopes and other media considerably narrower than the transfer-roller. This toner transfer is usually due to incorrectly charged toner and/or unintended development such as during paper misfeeds. As a result, relatively small amounts of toner, e.g., between about 1% and 3% of the so-called background, end up in the gap regions and, in turn, on the transfer-roller. Eventually, a significant amount of toner is captured on the roller, typically in stripe form, causing unwanted soiling of the paper being fed and shortened roller life.
While undesirable, this occurrence was not generally considered problematic for several reasons. First, toner coverage in the unprinted or background areas is frequently quite low. Second, the transfer voltage could be turned off between sheets of paper, then on again, preventing, in theory, photoconductor to transfer-roller toner contact. Third, in applications such as printing on photographic paper where toner coverage is relatively high, such paper usually comes in a roll so that there are no gaps between sheets of paper. It is noted, in any case, that conventional printers are often readily modifiable for use of rolls rather than sheets of paper.
Although not an issue with every printing job, generally speaking, it is highly desirable to allow images to be printed to the edges of the paper. This is particularly true in photo paper printing applications which use clear toner to protect ink-jet prints. In these applications, a thick, clear toner layer about 20 microns thick, for example, is applied uniformly to the photo paper all the way to the edge thereof. The toner is then laid upon an ink-jet printed image and, subsequently, electrostatically fused to the photo paper to provide protection against moisture.
To insure coverage of the entire surface of the paper, it has been found that the photoconductor itself and, hence, the photoconductor toner layer must be generally wider than the photo paper, e.g., by several millimeters. Accordingly, where only one width of paper is used, a transfer-roller several millimeters narrower than the paper can be used without toner capture on the roller. Beyond each end of the transfer-roller, i.e., where the photoconductor (or equivalent) is wider than the paper, the toner has nowhere to go and is subsequently scraped from the photoconductor by a cleaning blade. Alternatively, where no cleaning blade is provided, toner on the photoconductor simply returns to the development region and the development roller reabsorbs the toner. Because photo paper is usually sufficiently stiff and adequately conductive to receive the desired charges from the charge roller, for optimum results, toner should be transferred uniformly to the outward most edges of the photo paper, even where there is no direct support from the transfer-roller.
Problems frequently arise, however, where multiple paper widths are utilized. Specifically, wherever the paper is narrower than the transfer-roller (and the photoconductor), some toner usually develops on the transfer-roller. This problem is magnified in that the development density present is at least twice that of solid area development densities, such as background toner levels, typically transferrable in the space between paper sheets. In some cases, up to about 2 mg/cm2 may be continuously placed on the transfer-roller, resulting in relatively heavy stripes of toner on those portions of the roller adjacent to the paper edges.
Hence, when paper of greater width is subsequently printed upon, toner buildup from previous, smaller width printing jobs reduces the resiliency of the transfer-roller at the point of contact between the roller and paper. This, in turn, results in variable contact pressure, particularly in areas away from the stripes, leading to uneven transfer of toner in regions outside the striped areas. It also causes transfer of toner onto the back side of the paper, despoiling the paper and hindering fusion of the toner thereto.
A system and a method are, therefore, desired for improving the quality of printed images on printing media as the geometry of the media changes, so as to provide not only even, uniform coverage of toner and the corresponding image to be printed, but also to the physical edges of the media, simply, practically, reliably and at a relatively low cost.
Accordingly, a specific, illustrative apparatus is provided for use in a system including a photoconductor for collecting a photostatic charge in a selected form corresponding to an image to be printed, and for retaining toner in the form of the image on a first surface of printing media. The apparatus includes a transfer roller in contact with the photoconductor at a selected transfer point for transferring the toner image from the photoconductor to the first surface of the printing media. The transfer roller has at least one groove in proximity to each roller end for accommodating overhanging edges of printing media as a setback from the paper edges and for clearance from toner on the photoconductor.
Another specific, illustrative apparatus is provided for use in a system including a photoconductor for collecting a photostatic charge in a selected form corresponding to an image to be printed, and for retaining toner in the form of the image on a first surface of printing media. The apparatus includes a transfer roller in contact with the photoconductor at a selected transfer point for transferring the toner image from the photoconductor to the first surface of the printing media. The transfer roller has extendable roller ends such that, upon their extension a selected distance, at least one groove is formed in proximity to each roller end for accommodating overhanging edges of printing media as a setback from the paper edges and for clearance from toner on the photoconductor.
A further specific, illustrative apparatus is provided for use in a system including a photoconductor for collecting a photostatic charge in a selected form corresponding to an image to be printed, and for retaining toner in the form of the image on a first surface of printing media. The apparatus includes a plurality of transfer rollers arranged about a rotatable carrousel. Each roller is rotatably mounted to the carrousel such that at least one of the rollers is positionable for operative engagement with the photoconductor at a selected transfer point, such engagement effecting transfer of the toner image from the photoconductor to the first surface of the printing media. Each roller has an effective length different than that of the other rollers so as to enable overhanging edges of printing media having a selected width, such overhanging edges providing a setback from the paper edges and clearance from toner on the photoconductor.
Yet another specific illustrative apparatus is provided for use in a system including a photoconductor for collecting a photostatic charge in a selected form corresponding to an image to be printed, and for retaining toner in the form of the image on a first surface of printing media. The apparatus includes a plurality of transfer rollers arranged along a translatable carriage. Each roller is rotatably mounted to the carriage such that upon selected translation of the carriage, at least one of the rollers is positionable for operative engagement with the photoconductor at a selected transfer point. Such engagement effects transfer of the toner image from the photoconductor to the first surface of the printing media. Each roller has an effective length different than that of the other rollers so as to enable overhanging edges of printing media having a selected width. Such overhanging edges provide a setback from the paper edges and clearance from toner on the photoconductor.
Also provided is a specific, illustrative system for printing toner or the like on printing media. The system comprises a drum for retaining toner, and a transfer roller in contact with the drum at a selected transfer point for transferring a uniform layer of the toner from the drum to a first surface of the printing media. At least one end of the transfer roller is sized and configured for accommodating overhanging edges of printing media as a setback from the paper edges and for clearance from toner on the drum. In addition, a power source is provided for applying a selected voltage between the drum and the transfer roller and for attracting the toner thereto, so as to effect transfer of the uniform toner layer from the drum to the printing media first surface.
Still another specific, illustrative system is provided for printing toner or the like on printing media. The system includes a development roller for retaining toner in a form of an image desired to be printed. A transfer roller, in contact with the development roller at a selected transfer point, effects transfer of a uniform layer of the toner from the development roller to a first surface of the printing media, the toner adhering to the first surface at the selected transfer point. At least one end of the transfer roller is sized and configured for accommodating overhanging edges of printing media as a setback from the paper edges and for clearance from toner on the drum.
In addition, a specific illustrative method is provided for printing an image electrophotographically on printing media. First, a photoconductor for collecting a photostatic charge is moved in a first direction. Next, using a selected voltage from a power source, the photoconductor is charged with the photostatic charge. Selected portions of the charge applied to the photoconductor are then activated, such portions corresponding to an image to be printed on the printing media. A development roller is rotated adjacent to and in tandem with the photoconductor but in a third direction generally opposite to that of the first, and toner is transferred from the development roller to the photoconductor, the toner photostatically adhering to the photoconductor in a form corresponding to the image to be printed. Thereafter, a transfer roller is placed in contact with the photoconductor, the point of contact defining a selected point for transferring toner from the photoconductor to a first surface of the printing media. The transfer roller has at least one groove in proximity to each roller end for accommodating overhanging edges of printing media as a setback from the paper edges and for clearance from toner on the photoconductor. Next, the printing media is transported in tandem with and to a point between the photoconductor and the transfer roller. Finally, toner on the photoconductor is attracted toward the transfer roller so as to effect transfer of the toner image from the photoconductor to the printing media first surface.
Another specific illustrative method is provided for printing an image electrophotographically on printing media. Initially, a photoconductor for collecting a photostatic charge is moved in a first direction. Using a selected voltage from a power source, the photoconductor is then charged with the photostatic charge. Next, selected portions of the charge applied to the photoconductor are activated, such portions corresponding to an image to be printed on the printing media. A development roller is then rotated adjacent to and in tandem with the photoconductor but in a third direction generally opposite to that of the first. Toner is, in turn, transferred from the development roller to the photoconductor, the toner photostatically adhering to the photoconductor in a form corresponding to the image to be printed. Next, a transfer roller having extendable ends is placed in contact with the photoconductor, the point of contact defining a selected point for transferring toner from the photoconductor to the first surface of the printing media. The ends of the transfer roller are extended a selected distance such that at least one groove is formed in proximity to each roller end for accommodating overhanging edges of printing media as a setback from the paper edges and for clearance from toner on the photoconductor. The printing media is transported in tandem with and to a point between the photoconductor and the transfer roller. Finally, toner on the photoconductor is attracted toward the transfer roller so as to effect transfer of the toner image from the photoconductor to the printing media first surface.
In a further specific, illustrative method for printing an image electrophotographically on printing media, a photoconductor for collecting a photostatic charge is first moved in a first direction. Using a selected voltage from a power source, the photoconductor is then charged with the photostatic charge. Next, selected portions of the charge applied to the photoconductor are activated corresponding to an image to be printed on the printing media. A development roller is then rotated adjacent to and in tandem with the photoconductor but in a third direction generally opposite to that of the first. Toner is, in turn, transferred from the development roller to the photoconductor, the toner photostatically adhering to the photoconductor in a form corresponding to the image to be printed. Next, a carrousel having a plurality of transfer rollers arranged thereabout is rotated, each roller being rotatably mounted thereto so as to position at least one of the rollers in contact with the photoconductor upon carrousel rotation. The point of contact defines a selected point for transferring the toner image from the photoconductor to a first surface of the printing media, each roller having an effective length different than that of the other rollers so as to enable overhanging edges of printing media having a selected width. Such overhanging edges provide a setback from the paper edges and clearance from toner on the photoconductor. The printing media is transported in tandem with and to a point between the photoconductor and the transfer roller. Finally, toner on the photoconductor is attracted toward the transfer roller so as to effect transfer of the toner image from the photoconductor to the printing media first surface.
Yet another specific illustrative method relates to printing an image electrophotographically on printing media. Initially, a photoconductor for collecting a photostatic charge is moved in a first direction. Using a selected voltage from a power source, the photoconductor is then charged with the photostatic charge. Next, selected portions of the charge applied to the photoconductor, corresponding to an image to be printed on the printing media, are activated. A development roller is rotated adjacent to and in tandem with the photoconductor but in a third direction generally opposite to that of the first. Toner is, in turn, transferred from the development roller to the photoconductor, the toner photostatically adhering to the photoconductor in a form corresponding to the image to be printed. Next, a carriage with a plurality of transfer rollers rotatably mounted thereto is translated so as to position at least one of the rollers in contact with the photoconductor. The point of contact defines a selected point for transferring the toner image from the photoconductor to a first surface of the printing media. In addition, each roller has an effective length different than that of the other rollers so as to enable overhanging edges of printing media having as elected width, such overhanging edges providing a setback from the paper edges and clearance from toner on the photoconductor. The printing media is then transported in tandem with and to a point between the photoconductor and the transfer roller. Finally, toner on the photoconductor is attracted toward the transfer roller so as to effect transfer of the toner image from the photoconductor to the printing media first surface.
Still another specific, illustrative method is provided for printing toner or the like on printing media. Initially, a drum for retaining toner is rotated in a first direction. A transfer roller is then rotated in a second direction generally opposite to that of the first. The transfer roller is in contact with the drum at a selected transfer point for transferring a uniform layer of the toner from the drum to a first surface of the printing media. At least one end of the transfer roller is sized and configured for accommodating overhanging edges of printing media as a setback from the paper edges and for clearance from toner on the drum. Using a selected power source, a selected voltage is applied between the drum and the transfer roller for attracting the toner thereto so as to effect transfer of the uniform toner layer from the drum to the printing media first surface.
Yet a further specific, illustrative method is provided for printing toner or the like on printing media. First, a development roller for retaining toner in the form of an image desired to be printed is rotated in a first direction. A transfer roller is then rotated in a second direction generally opposite to that of the first. The transfer roller is in contact with the development roller at a selected transfer point for transferring a uniform layer of the toner from the development roller to a first surface of the printing media. The toner adheres to the first surface at the selected transfer point, at least one end of the transfer roller being sized and configured for accommodating overhanging edges of printing media as a setback from the paper edges and for clearance from toner on the drum.
The present invention will now be further described by reference to the following drawings which are not intended to limit the accompanying claims.